1. Field of the Invention
An optimum excitation frequency, which optimizes production of satellite ink drops (very small ink drops compared to the main ink drops) and stability of the ink disintegration phase, is automatically determined and relates also to an optimum excitation frequency setting method for the continuous jet type ink jet recording apparatus.
2. Description of the Related Art
Various Ink jet recording apparatus of the continuous jet type are conventionally known and practically used. An exemplary one of such conventional continuous jet type ink jet recording apparatus is disclosed, for example, in Japanese Patent Laid-Open Application No. Heisei 4-220350 and shown in FIG. 10. As presented in FIG. 10 the continuous jet type ink jet recording apparatus shown includes, as principal components thereof, a nozzle 1 having a circular orifice of a very small diameter, an ink electrode 2 for holding the potential of ink in the nozzle 1 at the ground level, an oscillator 3 in the form of a piezoelectric oscillator mounted on the nozzle 1, a control electrode 4 having a circular opening or a slit-like opening coaxial with the nozzle 1 and connected to receive a charge controlling signal to control charging of a jet of ink in accordance with image data, a grounding electrode 5 disposed in the rear (rightwardly in FIG. 10) of the control electrode 4 and grounded itself, a knife edge 6 mounted on the grounding electrode 5, a deflection power supply E1, a deflection electrode 7 connected to the deflection power supply E1 for cooperating with the grounding electrode 5 to produce therebetween an intense electric field perpendicular to an ink jet flying axis to deflect a charged ink drop to the grounding electrode 5 side, a reference oscillator CG for generating a reference clock signal CLK of an oscillation frequency instructed from a microprocessor unit (hereinafter referred to simply as MPU) not shown, a frequency divider FD for dividing the frequency of the reference clock signal CLK by N (positive integer) to produce an excitation signal PCLK, a delayed pulse generator DG for delaying the excitation signal PCLK to produce excitation signals PCLK of phases .theta..sub.0, .theta..sub.1, .theta..sub.2, . . . , .theta..sub.N-1 delayed to N (positive integer) stages in response to the reference clock signal CLK, a multiplexer MP for selecting one of the excitation signals PCLK of the thus delayed phases .theta..sub.0, .theta..sub.1, .theta..sub.2, . . . , .theta..sub.N-1, an oscillation element driver VD for driving the oscillator 3 with the excitation signal PCLK of the phase .theta. selected by the multiplexer MP, a pulse width modulator PM for converting image data into a pulse width signal corresponding to a density gradation, a synchronizing circuit SC for synchronizing a rising or falling edge of the output of the pulse width modulator PM with a rising or falling edge of the excitation signal PCLK from the frequency divider FD, and a high voltage switch HVS for voltage amplifying and applying the output of the synchronizing circuit SC as a charge controlling signal to the control electrode 4. It is to be noted that reference character DR denotes a rotary drum around which a recording medium is wrapped.
In the conventional continuous jet type ink jet recording apparatus of the construction described above, the MPU variably sets the oscillation frequency of the reference clock signal CLK of the reference oscillator CG to variably set the excitation frequency of the excitation signal PCLK. The picture quality of a result of recording depends upon the ink disintegration characteristic of the nozzle 1, and the ink disintegration characteristic varies depending upon the excitation frequency of the excitation signal PCLK. Consequently, in the conventional continuous jet type ink jet recording apparatus, test images are printed successively varying the excitation frequency of the excitation signal PCLK and are checked for the picture quality by visual inspection, and an optimum excitation frequency is manually set from the outside (by means of an operation panel or the like) based on the visual check.
The conventional continuous jet type ink jet recording apparatus described above has two problems in terms of a manner of disintegration of an ink jet into ink drops from an ink column.
The first problem relates to a satellite drop which is produced between principal drops by the non-linearity of the surface deformation of an ink column. Three different modes are possible in regard to production of a satellite drop including a mode wherein a satellite drop produced is integrated with a succeeding principal drop, another mode wherein a satellite drop produced is integrated with a preceding principal drop, and a further mode wherein a satellite drop is not integrated until it comes to a recording surface. In continuous jet type ink jet recording apparatus, a still further mode wherein no satellite drop is produced is desirable. However, even if a satellite drop is produced, if it is integrated rapidly in the control electrode 4, it does not matter especially. However, when the integration occurs so late that a satellite drop is integrated in the rear of an exit of the control electrode 4 or no integration of a satellite drop occurs until it comes to a recording surface, the charged satellite drop (whose specific charge is usually higher than that of a charged principal drop) is influenced much by a deflection electric field and is deflected precedently to a charged principal drop. As a result, an electrostatic repulsive force from the satellite drop acts in a perpendicular direction to the ink jet flying axis upon the charged principal drop to obstruct correct deflection of the charged principal drop. Further, if the charged satellite drop is deflected out of a correct trajectory and integrated with a non-charged principal drop to be recorded, the noncharged principal drop is deflected a little. Any of the events described above deteriorates the picture quality very much. This problem will be hereinafter referred to as satellite drop problem.
The second problem is that the disintegration phase in (timing at) which an ink column is disintegrated into an ink drop is different among individual ink drops and is not stabilized with respect to the phase of the excitation signal PCLK. This small dispersion in disintegration phase at a disintegration point is amplified by an influence of the resistance of the air during flying of the ink drop and appears as a large fluctuation in position in the proximity or rearwardly of the knife edge 6. Further, the phase of the charge controlling signal during printing is kept fixed with reference to the phase of the excitation signal PCLK. If the disintegration phase varies in this condition, then not only the charging phase cannot be kept optimally, but also the measurement of the optimum charging phase suffers from an error. Such jet is defined as fuzzy jet. A fuzzy jet gives rise to, similarly to the satellite drop problem described above, significant reduction in picture quality because of production of an intermediate charged ink drop due to a fluctuation of the position of an ink drop in the ink jet axial direction and a fluctuation of the charging phase. This problem is defined as fuzzy jet problem.
Empirically, the two problems described above depend much upon the excitation frequency of the excitation signal PCLK. In particular, it is considered that the two problems depend upon the frequency characteristic of a mechanical oscillation system by which oscillations of the oscillator 3 mounted on the nozzle 1 are transmitted to the disintegration point of an ink jet.
In order to solve the problems described above, in a conventional continuous jet type ink jet recording apparatus, test images are printed successively varying the excitation frequency of the excitation signal PCLK and are checked by visual inspection to select an optimum excitation frequency, and the actual excitation frequency is manually set to the optimum excitation frequency. Therefore, the conventional continuous jet type ink jet recording apparatus is disadvantageous in that it is cumbersome that test images must be printed actually, that a criterion is very indefinite since it depends upon subjective visual observation of images, that setting of an excitation frequency which has been determined to be optimum is cumbersome because it is performed manually, and so forth.